Internal Wall Insulation

Step-by-Step - Mineral Wool Insulation Fitted Between Timber Battens

Fibrous or “wool” insulation has several advantages over rigid foam boards. Most importantly it’s breathable, meaning it’s better at drying out if it gets damp. This makes it particularly suitable for older buildings with single-leaf brick walls which might be slightly porous. Mineral wool is also “squishy” which means it can be friction-fitted between wooden battens and you won’t need to cut it to precise shapes before fitting.

There are several types of wool insulation, the most common of which are mineral wool (also known as “rock wool”) and (fibre)glass wool. Natural wool insulations including sheep’s wool are also available but we wouldn’t recommend these because they can be vulnerable to damp and insects. Wool insulation is available in either roll or slab format.

N.B. Theres a “shopping list” at the bottom of this page with links to suitable products for this project.

Rockwool RWA45 industry-standard mineral wool slabs are used in this example. 100mm thickness is required to achieve a U-value of 0.3 as required by Building Regulations.

Step 1 - Preparation

Remove any items fixed to the external walls (radiators, light switches, plug sockets, skirting boards, curtain rails, etc.) Strip off the existing plaster, back to bare brick. If there are any gaps or cracks these should be repaired before proceeding.

N.B. It’s possible to install new insulation on top of existing plaster so long as it’s dry and in good condition. However, it’s often difficult to tell if there are hidden defects or water penetration without removing the plaster. If you have thick layers of existing plaster you will also give yourself a fraction more floorspace by removing it.

Step 2 - Attach Battens

Attach a grid of treated softwood battens to the wall. These should be 100mm deep by 50mm wide. Start with horizontal strips along the floor and ceiling then infill with vertical pieces at 600mm centres (leaving 550mm gaps). Also frame around any openings such as windows. Some also prefer to add horizontal pieces to support the individual slabs of insulation but this is optional.

The battens should be attached to the brickwork using suitable screws (150mm long) and masonry fixings at no more than 600mm centres. It’s worth drilling pilot holes to avoid splitting the timber. Holes can be drilled transversely through the battens for electrical cables and notches cut around water or gas pipes.

The battens should be installed as vertical as possible. A good laser lever will be essential for this purpose. If the walls are irregular you might have gaps behind the battens in some places. The gaps should be packed with timber shims and/or filled with low-expansion spray foam before screwing in place.

Heavy items such as radiators, shelves and wall-mounted cabinets should be fastened directly into battens rather than hung from the plasterboard. The two closely-spaced battens beneath the window are positioned for re-fixing the radiation.

Step 3 - Install Insulation

Rockwool RWA45 is used in this example. This is a mineral wool product with good thermal and acoustic insulating properties. Importantly it’s also non-combustible.

The slabs are 100 thick, 600 wide and 1200 long. If you’ve positioned the studs accurately, the clear gaps should be 550mm, into which the Rockwool slabs can be compressed for a tight fit. The slabs can be cut to size with a kitchen knife if needed.

Remember to cut openings for any socket boxes.

Step 4 - Vapour Membrane

Due to the Rockwool’s breathability it’s recommended to cover it with a vapour control membrane to maintain airtightness and prevent water vapour condensing within the wall.

The membrane should be stapled to the battens at regular centres. Fixing the sheets horizontally will usually result in fewer joints to seal in the next step.

Step 5 - Seal Joints

To complete the airtightness, all joints should be sealed using a combination of expanding foam and sealant tape. Take special care to seal around any gaps such as socket boxes or pipes.

Step 6 - Window Reveal

Before fitting plasterboard, the window reveal should be insulated using sheets of thinner “reveal board” insulation such as extruded polystyrene or Aerogel. These are available with plasterboard already attached which simplifies the next step.

N.B. It might be simpler to complete this step before taping the joints so consider your specific arrangement and switch the order if necessary.

Step 7 - Plasterboarding

The next step is to screw plasterboard to the battens using drywall screws at 600mm centres. Cover the joints with “scrim” tape and reinforce external corners (e.g. around the window) with galvanised angle beads.

Step 8 - Finishing

The wall can now be plastered and painted. Window sills, skirting boards, radiators and other fixings can now be reinstated.

Lime plaster is often recommended by conservationists for its breathability, however for a thin “skim coat” like this regular gypsum plaster is usually sufficifiently breathable and removes the need for specialist materials or skills.

Water-based paints are preferable to oil-based ones because they are more permeable.